In the production of foamed products, polyurethanes have been found to be highly advantageous because of their versatility and their ability to be matched to the production requirements and the product requirements. As a result, foamed polyurethanes have achieved world-wide significance in the industrial polymer field. The production of polyurethanes utilizes a variety of techniques.
One of the most important factors in modern polyurethane technology is the need to avoid the use of fluorochlorohydrocarbons as expanding or foaming agents, because of the detrimental effect of such compounds upon the environment. More expensive replacement products may be uneconomical.
The use in the past of halogen-free foaming agents, especially carbon dioxide on ecological grounds, has been found to be desirable although economic factors have hitherto made such use less than attractive. In spite of considerable research, the use of carbon dioxide frothing as a replacement for fluorochlorohydrocarbon foaming has not been found to be fully satisfactory as will be apparent from M. Taverna, Conference paper UTECH 1990, pages 70-73.
Problems have also been encountered with other expanding gases used in foaming, particularly with respect to changes in polyurethane technology which are required when such gases are used. For example, an improvement in product quality is required, for instance to drastically increase the life span of the molded products which are produced. It is also necessary to improve the load-carrying capacities of such products and the aging characteristics thereof.
Apart from quality factors, a reduction in the problem with respect to fumes and fogging in the industrial application of polyurethanes, which also can affect the consumer, is necessary. For example, efforts have been required to ensure that all of the additives which are not chemically fixed in the polymer matrix should be removed or eliminated to avoid the fume or fogging problems. As a practical matter, therefore, aspects of workplace hygiene, toxicology and ecology have affected the industry detrimentally. Nonreacted isocyanates, phthalates serving as solvents for certain additives, partly volatile metallo-organic compounds and even certain amine catalysts have contributed to these problems.
However, the efforts to avoid volatile components as much as possible during production and in the finished product have not resulted in qualitative improvements in the products. They have been only limitedly successful in workplace improvements from the aspect of hygiene, toxicology and ecology. Nevertheless considerable efforts are being expended in this aspect of foam plastic technology.
There is a considerable world-wide effort under way to find new raw materials with the aid of which the aforementioned problems in foam technology can be resolved. For example, considerable effort is under way to develop high molecular weight polyols as starting materials for polyurethane foam technology.
In more recent technical literature, a trend can be discerned toward the use of polyether amines as reaction components for use in the production of polyurethanes. By contrast to classical polyols which have at least two free hydroxyl groups per molecule, the polyether amines have, instead of free hydroxyl groups or residues, primary or secondary aliphatic amine or primary aromatic amine residues.
In R.D. Priester, R.D. Peffley and R.B. Turner; Proceedings of the SPI-32nd Technical Marketing Conference, San Francisco, 1989, the use of aminopolyols with secondary aliphatic or primary aromatic amino functionality, is described for the production of foamed polyurea compositions of low bulk density. It has been shown that these materials have significant advantages by comparison with conventional polyurethane foam components in many ways.
These advantages include increased load-carrying capacity and improved aging characteristics as well as an improvement in resistence to flammability.
In EP-A 0 279 536, long-chain aliphatic oligofunctional secondary amines are reacted with polyisocyanates in the presence of conventional catalysts and additives to produce polyurea foams which are not further characterized.
This technique is characterized by improvement in many of the requirements outlined above and thus is considered a true advance in the art. However, it does not satisfy the need to eliminate classical additives such as tin-containing compounds and amine catalysts which contribute to the toxicity and fogging problems noted above. Furthermore, the technique has a further disadvantage which has limited its applicability, namely, the problem of obtaining the polyether amines. Up to now the production of polyether amines by simple methods which are both economically and technologically suitable for widespread use in polymer technology has not been possible. These advantages gave rise to high raw material cost, to output problems and/or created problems with guaranteeing the chemical integrity of the resulting product.
The production and use of mixtures of polyether amines having secondary amine functional groups in the production of foamed polyurea compositions is described at many locations in the literature. Large-chain aliphatic oligofunctional primary amines (U.S. Pat. No. 3,654,370) can be subjected to reaction by alkoxylation, cyanethylation, alcohol aminylation and reductive catalytic aminylation.
Alternatively, short-chain primary aliphatic amines can be subjected to a catalytic reaction with polyoxyalkylene compounds (U.S. Pat. No. 3,654,370) to long-chain aliphatic oligofunctional secondary amines (U.S. Pat. No. 4,904,705).
It has been proposed further to produce aromatic polyether amines in a variety of processes, for example, by reacting commercially-available polyhydroxy polyethers with isatic acid anhydride in one or more steps. In this sense, DE-A 29 48 419 describes the transformation of commercially-available polyhydroxy polyethers with aromatic diisocyanates into prepolymers, with subsequent hydrolysis of the remaining isocyanate products to yield an aminopolyol with terminal amino groups bound to aromatic residues.
In the polymer chemical field it is known that the product characteristics of polymers can be better controlled and influenced if the raw materials used are homogeneous and invariant. This applies as well for the formulation and product characteristics of polyurethane polymers under discussion. It is, therefore, important to avoid statistical distribution and differently substituted amines in the raw materials employed and this can be achieved by utilizing reactive terminal groups.
With the teachings of DE-A 38 25 637, albeit at increased cost, it is possible to obtain highly uniform and invariant, long-chain aliphatic oligofunctional secondary amines. This is achieved by hydrogenation of Schiff bases produced from ketones and oligofunctional primary polyoxypropylene amines.
Oligofunctional polyoxypropanol amines with terminal primary amine groups are used in accordance with DE-A 38 25 637 to produce polyether amines with secondary amino groups and are commercially available under the designation JEFFAMINE.RTM. (Texaco). The molecular weight of this starting material is in the range of 230 to 8,000 and its amine functionality lies between 1 and 3.
These compounds are produced, for example, as described in U.S. Pat. No. 3,654,370, in a single step from commercially-available petrochemical compounds, namely, dioxypropylenepolyols and ammonia.
A direct use of such products for the production of foamed polyurea compositions of low bulk density is advantageous on both economical and technological grounds because these compounds have an especially high degree of amination and high compositional consistency with respect to their terminal groups. It is also an advantage that the formation of polyol compounds does not result in a significant reduction of esters, urethanes or ethoxy residues; indeed these residues can be completely avoided in most instances.
In the literature, the use of aliphatic polyether amines with primary amino function is described for the production of hard elastic foamed polyurea compositions of high bulk density. In fact, the literature teaches that the extremely high reactivity of these polyether amines precludes the formation of foamed polyurea compositions of low bulk density.
In the production of "foamed" polyurea compositions with such high bulk densities (800 to 1300 kg/m.sup.3), the reaction times of a maximum of 2 to 3 seconds must be observed with the use of such primary oligofunctional long-chain aliphatic amines, because the liquid raw material solidifies in such a short time into solid and no longer flowable masses.
In EP-A 0 081 701 and U.S. Pat. No. 4,269,945, the addition of a blowing or foaming agent to this process is described. The result is an improvement in the product characteristics, namely, the surface quality of the hard elastic foam body which results. With this approach, microcellular "foams" can be obtained with a high bulk density of greater than 800 kg/m.sup.3. In this process, the use of permanent dried gases, especially nitrogen or air, has been found to be advantageous. The process has been described as "nucleation". While in connection with this approach, the use and/or collateral effect of autogenously produced carbon dioxide has been proposed, it has not resulted in any practical applications because it has been accompanied in the past with an extremely rapid reaction rate predominantly causing the reaction mixture to harden into a nonflowable mass. The velocity of the reaction is far more rapid than the much slower catalytic reaction of water with isocyanates.
U.S. Pat. No. 4,910,231 describes the reaction of a primary amino group containing polyether amines with an excess of polyisocyanate and water to form a hard foam which is not unlike the hard foams described with their disadvantages.